US defense secretary says US "invested in capabilities to ensure that Iranian attempt to close down shipping" can be stopped.

WASHINGTON - The United States has the military capacity to defeat any Iranian attempt to shut down sea commerce in the oil-rich Gulf region and will hold Tehran directly responsible for shipping disruptions, Defense Secretary Leon Panetta said on Wednesday.

"The United States is fully prepared for all contingencies here," Panetta told a Pentagon news conference. "We've invested in capabilities to ensure that the Iranian attempt to close down shipping in the Gulf is something that we are going to be able to defeat, if they make a decision to do that."

Panetta's comments came amid rising tensions in the region over Iran's nuclear program and its threats to close the vital Strait of Hormuz unless the international community revokes sanctions imposed on Iranian oil shipments.

A security team aboard a US Navy oiler opened fire with a .50 caliber machine gun on a small boat off the United Arab Emirates this week after it failed to heed warnings to alter its course. An Indian fisherman was killed and three others were wounded in the incident, which is under investigation.

Concerns about the security situation in the region have prompted the Pentagon to bolster its military presence, announcing this week, for example, that it would move the USS Stennis aircraft carrier strike group to the Gulf four months earlier than planned in order to maintain current force levels.

The Stennis will replace another carrier that is being withdrawn, which would have left one strike group in the region to counter Iran and support US combat air operations in Afghanistan.

The US military announced on Tuesday more than 20 countries would participate in anti-mine exercises in the region in September. The exercises will not take place in the Strait and the Pentagon denied it was sending a message to Iran.

The USS Ponce, a forward staging base ship, arrived in the Gulf earlier this month to help support mine countermeasure operations and other maritime security efforts in the region, the military said.

British Defense Secretary Philip Hammond, appearing at the Pentagon news conference with Panetta, said London was committed to helping ensure Gulf sea lanes remained open.

"I can reaffirm our commitment to play our part in maintaining freedom of navigation in the international waters of the Gulf and the Straits of Hormuz," Hammond said. "Any attempt by Iran to close the Straits would be illegal and the international community will not allow it to happen."

Panetta said Washington was prepared to respond to any move against sea commerce by Tehran.

"The Iranians need to understand that the United States and the international community are going to hold them directly responsible for any disruption of shipping in that region, by Iran or for that matter by its surrogates," he said.